High bypass turbofan engines are widely used for high performance aircraft which operate at subsonic speeds. Large fans for high bypass turbofan engines are typically placed at the front of the engine, and serve to produce greater thrust and reduce specific fuel consumption.
It is known in the art that a high number of fan blades in such engines is advantageous in that it reduces the overall system weight of the engine. With higher blade counts, the blade width, or chord, must be reduced to accommodate more blades. To maintain the airfoil contour, the thickness of each blade must also be reduced, resulting in a significantly lighter blade. Fan design criteria is such that the total weight of the blades will decrease with greater numbers of lighter blades.
Due to having thinner blades, high bypass turbofan engines having high blade counts, e.g., in excess of about 28 blades, must incorporate part span shrouds which provide added stiffness for frequency and stability control. The shroud serves to increase the natural frequencies of the blades, while also promoting blade stability control to counteract a self-excited vibrational instability termed "flutter". Without shrouding, the number of fan blades must be reduced to allow wider and thicker, and thus stiffer blades, so as to keep their natural frequencies in flexure between the prime excitation frequencies of the engine.
For a fixed root blade configuration (i.e., the blade is dove-tailed into the hub), a typical shrouded fan rotor has about 32 to 38 blades, while a typical unshrouded fan rotor is limited to about 22 to 26 blades. For blade counts of 28 to 30, a pinned root blade configuration (i.e., each blade pivots about an axis which is parallel to the engine's axis) generally is able to provide frequency control without the requirement for a shroud. However, a pinned root blade configuration demands a higher radius ratio, i.e., the radius of the hub to the radius of the blade tips, requiring a larger fan diameter to produce a given thrust. As with lower blade counts, a larger fan diameter incurs additional and undesirable weight.
A disadvantage with the use of shrouds is that shrouds tend to reduce engine performance by interfering, to some degree, with air flow through the fan. This effect is more pronounced in engines having high tip speeds of about 1350 to about 1600 feet per second. Consequently, low blade counts are typically required for such engines. To offset the weight penalty incurred by the low blade count, these engines generally require hollow fan blades made from materials such as titanium alloys. However, the weight savings derived from the use of hollow blades is diminished in smaller engines, since the smaller blades permit proportionally smaller hollowed regions within the blade. In addition, hollow blades are more expensive to manufacture than solid blades.
In very high bypass fans, composite fan blades formed from such composite materials as graphite epoxy are used to offset weight increases with lower blade counts. However, composite blades are brittle in comparison to titanium blades, and will necessitate thicker blades for small engines to withstand equivalent impacts. In addition, composite blades typically must also be thicker for increased fan tip speeds. Though additional weight is incurred by these requirements, composite blades are still generally lighter than hollow titanium blades for very high bypass engine applications.
From the above, it can be seen that, in terms of maintaining blade frequency and stability control, a conflict exists between engine performance and blade weight. Specifically, lower blade weight is achieved by higher blade counts, which necessitate a shroud to maintain blade frequency and stability. Furthermore, smaller engines that are limited to low blade counts will be proportionally heavier due either to the use of small hollow blades, which are less effective in reducing weight, or to the use of thicker composite blades. Therefore, in terms of minimizing weight, hollow blades do not provide a completely satisfactory solution for smaller engines, nor do composite blades for high tip speed bypass engines.
Accordingly, it would be advantageous to provide a means for increasing blade counts in high bypass turbofan engines, while providing frequency and stability control of the fan blades without the requirement for conventional part span shrouds. In addition, it would be preferable if such a means could offer an alternative to hollow blades on smaller engines, as well as composite blades for high tip speed bypass engine applications, without incurring significant additional weight. Further, it would be advantageous if such a means were sufficiently rugged and cost competitive with current technology blades.